Use of freeze-drying technique to make ultra-fine oxidizer for use in solid propellants



United States Patent Oflice Int. Cl. F26b /06 US. Cl. 34-5 6 Claims ABSTRACT OF THE DISCLOSURE The application of freeze-drying to an oxidizer such as ammonium perchlorate to obtain an ultra-fine oxidizer. The oxidizer thus produced is added to the other propellant ingredients normally used in a solid propellant and results in a solid propellant with an increased burning rate.

Dedicatory clause The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to me of any royalty thereon.

Background of the invention This invention relates to a novel method for making an ultra-fine oxidizer for use in solid propellants.

Modern rockets require increased thrust. Thrust can be increased in many ways. One of these ways is to increase the surface area of the oxidizer used in the propellant. This will increase the burning rate and thrust. It can be shown mathematically that an increase in the surface area can be accomplished by decreasing the average particle size of the oxidizer. These methods have included the use of mechanical devices such as grinders and ball mills. However, these methods are very hazardous and serious accidents have occurred as a result of their use in the propellant industry. Additionally, the lowest particle size obtained by these methods is not sufiiciently small.

Accordingly, the principal object of this invention is to provide a safe and improved method of obtaining an ultrafine oxidizer for use in solid propellants.

A particular object of this invention is to produce ultrafine ammonium perchlorate by freeze-drying.

Summary of the invention The method of this invention employs the application of freeze-drying to an oxidizer such as ammonium perchlorate to obtain an ultra-fine oxidizer. This involves rapidly subliming ice from a dilute solution of ammonium perchlorate in water. The oxidizer thus produced is added to the other propellant ingredients normally used and results in a solid propellant with an increased burning rate.

Description of the preferred embodiment This specification is based on the theory that the increased burning rate of a propellant is due to the increased surface area of the oxidizer. However, some authorities believe it is due to the more intimate contact of the oxidizer with the other propellant ingredients. Whichever theory is followed, the empirical result is the same.

The method of this invention employs the freeze-drying of an oxidizer. This involves first making a solution of any size and shape solid oxidizer in a solvent carrier that can be quick-frozen. The processing of the oxidizer in solution is particularly desirable in the propellant art from a safety standpoint.

3,452,445 Patented July 1, 1969 The preferred embodiment of this invention employs water as the solvent carrier. Water is preferred as the solvent carrier because of its low cost. The preferred embodiment of this invention employs ammonium perchlorate as the oxidizer.

The water solution of ammonium perchlorate can be quick-frozen by any one of several techniques. The best technique obtained during experimental studies involved placing 35 ml. of an 8% (by weight) ammonium perchlorate in distilled water solution into a 250 ml. round bottom flask. The flask is then placed on a rotating stirrer and rotated rapidly through a Dry Ice-acetone bath. This results in the solution freezing rapidly in a thin shell on the walls of the flask.

The frozen solution is placed under very low pressure (50-100 microns of mercury) and the ice is sublimed off. This method of vacuum drying is highly desirable in this field since it minimizes the danger of explosion and produces very fine perchlorate particles. The product is a very fine powder of extremely small particle size. The size is controlled by the conditions employed. These conditions are the concentration of the ammonium perchlorate in water, the speed of freezing and the method of freezing used.

It has been found experimentally that the optimum concentration of the ammonium perchlorate in water is about 8% to about 10% by weight. Average particle sizes from 1.7 to 2.1 microns have been obtained. The best average particle size obtained to date has been 1.7 microns (using a concentration of about 8% by weight). However, it is believed that improvements in laboratory and commercial techniques and equipment will eventually result in an average particle size of one micron. Present grinding techniques will not produce this size. If desired, larger particle sizes may be obtained by this method by increasing the concentration of the ammonium perchlorate in the water solution.

It is to be noted that the average particle size is not the only measurement to be considered in evaluating the eventual improvement in burning rate. The size distribution is also important. A lower average particle size is not an improvement if accompanied by an irregular size distribution. Thus, it is seen that the use of freeze-drying to produce an ultra-fine oxidizer avoids the dangers of the other methods used in the propellant field to produce small oxidizer particles.

Various modifications and variations of this invention will be come readily apparent to those skilled in the art in the light of the above teachings, which modifications and variations are within the spirit and scope of this invention.

What is claimed is:

1. An ultra-fine ammonium perchlorate product produced by the method of quickly freezing an ammonium per-chlorate and water solution containing from about 8% to about 10% ammonium perchlorate by weight, and sub liming off the ice for-med from the result of freezing by pulling a very low pressure on the solution.

2. The ultra-fine ammonium perchlorate product produced as defined in claim 1, wherein said pressure is from about 50 to about microns of mercury.

3. The ultra-fine ammonium perchlorate product produced as defined in claim 1, wherein said product has average particle sizes from about 1.'7 to about 2.1 microns.

4. The ultra-fine ammonium perchlorate product produced as defined in claim 1, wherein said pressure is from 50 to 100 microns om mercury, and said solution contains about 8% ammonium perchlorate by weight to produce a product with an average particle size of about 1.7 microns.

5. The ultra-fine ammonium perchlorate product as defined in claim 1, wherein said solution is quickly frozen by rotating and stirring rapidly in a Dry Ice-acetone bath, thereby forming a frozen solution.

6. The method of obtaining finely divided ammonium perchlorate particles by rapidly freeze drying a solution of ammonium perchlorate in water, said solution containing from about 8% to about 10% ammonium perchlorate by weight and being rotated and stirred rapidly in a Dry Ice-acetone bath to form a frozen solution, and subliming off the ice thereby formed to leave ultra-fine ammonium perchlorate of considerably reduced particle size to that used in making said solution, said sublimation occurring under a very low pressure from about 50* to about 100 microns of mercury.

UNITED STATES PATENTS 8/1934 Elser 345 X 12/1936 Reichel 34-5 X '6 1939 Flosdorf 34 5 X 12/1940 Flosdorf. 34-5 7/1954 Bower 345 X 5/ 1959 Leviton 345 7/1965 Rieutord 62345 X US. Cl. X.R. 

